1. Field of the Invention
The present invention relates to a charge coupled device ("CCD") and more particularly, to a CCD having microlenses and a method of manufacturing the CCD, in which the radius of curvature of the microlenses in the major axis direction is substantially the same as the radius curvature in the minor axis direction.
2. Description of the Related Art
FIG. 1 is a layout diagram of a general CCD.
Generally, a CCD is used to convert an optical picture into an electrical signal. As shown in FIG. 1, a general CCD is composed of a plurality of photo diode (PD) regions arranged in a matrix form and spaced apart by a set distance for converting a signal of light into an electrical signal; a plurality of vertical charge coupled device (VCCD) regions formed between the photo diode regions and arranged in vertical lines for transferring the electrical signal in a vertical direction; a horizontal charge coupled device (HCCD) region for transferring the electrical signal transferred from the VCCD regions in a horizontal direction; and a sensing amplifier (SA) for sensing the electrical signal transferred from the HCCD region.
Although not shown in FIG. 1, the general CCD further includes a color filter layer and a microlens formed on each of the photo diode regions for enhancing the performance of the CCD.
FIG. 2 is a cross-sectional view generally showing the structure of the general CCD of FIG. 1 along line II-II'.
As shown in FIG. 2, the general CCD includes photo diode regions 2 for converting a signal of light into an electrical signal. The photo diode regions 2 are arranged in a matrix form in a surface of a semiconductor substrate 1.
Between the columns of photo diode regions 2 arranged in the matrix form, vertical charge coupled device (VCCD) regions 3 are formed in the column direction for transferring the electrical signal from the photo diode regions 2 in a vertical direction. A metal shading layer 3 is formed on the VCCD regions 3, and a first planarizing layer 5 is formed on the entire surface including the metal shading layer 4 to planarize the top surface.
On the first planarizing layer 5, color filter layers 6 (first, second and third dye layers) are formed, and a second planarizing layer 7 is formed thereon. On the second planarizing layer 7, microlenses 8 are formed such that the microlenses 8 are convex toward a light source and not toward the photo diode regions 2.
In the general CCD, each of the microlenses 8 corresponds to one of the photo diode regions 2 so that all the light is directed to the photo diode regions 2.
FIGS. 3a, 3b-1 and 3b-2 are views for illustrating focusing effects of a light in the general image sensing device, such as the CCD shown in FIG. 2.
As shown in FIG. 3a, the microlenses 8 are arranged in a matrix form in the general CCD. Each of the microlenses 8 corresponds to one of the photo diode regions 2. Each microlens 8 has an oblong configuration due to the structure of photo diode regions 2 and VCCD regions 3.
FIG. 3b-l shows a focusing effect of a light impinging on the microlens 8 along the major axis (line A-A'), and FIG. 3b-2 shows a focusing effect of a light impinging on the microlenses 8 along the minor axis (line B-B').
As shown in FIGS. 3b-1 and 3b-2, the focusing distances of the light along the major axis and the minor axis of the microlens 8 are clearly different from each other. Accordingly, the general CCD has the following problems.
In the general CCD, microlenses are formed by patterning a photosensitive film through exposure and development process. The photosensitive is patterned in a rectangular form according to the cell configuration using a mask which is divided into a shading region and a transmitting region. A heat treatment is performed on the photosensitive film to form the microlenses. However, since the microlenses have a rectangular shape, the radius of curvature of the microlenses in the minor axis direction is substantially less than the radius of curvature of the microlenses in the major axis. As the radius of curvature of the microlens decreases, the focal distance of the microlens decreases.
The microlenses of the general LCD cannot focus the received light on the photo diode regions because of the large difference between the radii of curvature of the microlenses in the lateral direction and in the longitudinal direction. In fact, a portion of the received light is focused on other regions, e.g., an aluminum (Al) shading layer formed on the VCCD regions. As a result, in the general CCD, a relatively large loss of light and a deterioration in the resolution of the device results.
The structure of a conventional CCD and a method of manufacturing the conventional CCD for solving the aforementioned problems will be described below with reference to the attached drawings.
FIG. 4 is a cross-sectional view showing the structure of a conventional CCD.
In the conventional CCD as shown in FIG. 4, photo diode regions 12 for converting a light signal into an electrical signal are arranged in a matrix form. The photo diode regions 12 are spaced apart by a predetermined distance in the surface of a semiconductor substrate 11.
Between the photo diode regions 12, vertical charge coupled device (VCCD) regions 13 are formed for transferring the electrical signal in a vertical direction. On the VCCD regions 3, a metal shading layer 14 is formed for shielding the light from regions except the light-receiving regions of the device. An insulating film 15 for passivation and a first planarizing layer 16 are sequentially formed on the entire surface including the metal shading layer 14. Color filter layers 17 (first, second and third dye layers) are formed on the first planarizing layer 16 to transmit only the specific wavelengths of the light. A second planarizing layer 18 is formed on the entire surface including the color filter layers 17, and block stripe patterns 19 corresponding to the photo diode regions 12 are formed. On the block stripe patterns 19, microlenses 20a are formed. Each of the microlenses 20a corresponds to one of the photo diode regions 12.
A method of manufacturing the conventional CCD of FIG. 4 will be described below.
FIGS. 5a and 5b are cross-sectional views for illustrating a method of manufacturing the conventional CCD of FIG. 4.
To begin with, as shown in FIG. 5a, the photo diode regions 12 are formed in a matrix form and are spaced apart by a predetermined distance in the upper surface of the semiconductor substrate 11.
Between the columns of photo diode regions 12, VCCD regions 13 are formed in the column direction to transfer an electrical signal in a vertical direction. The metal shading layer 14 is selectively formed on the VCCD regions 13 but not on the photo diode regions. The insulating film 15 as a passivation layer is formed on the entire surface including the metal shading layer 14, and the first planarizing layer 16 is formed thereon.
A photoresist (not shown) with dyeability is coated on the first planarizing layer 16 and patterned through exposure and development process. Using a dyeing apparatus, dyeing is performed on the patterned photoresist to form a first dye layer of the color filter layers 17.
Using the same dyeing method, a second dye layer of the color filter layers 17 is formed on a predetermined portion of the first planarizing layer 16. Successively, a third dye layer of the color filter layers 17 is formed on a predetermined portion of the first planarizing layer 16 so as to overlap the second dye layer. The color filter layers 17 are then successfully formed on the first planarizing layer 16.
On the entire surface including the color filter layers 17, the second planarizing layer 18 is formed and coated with a sensitive resin having good transmissivity. The sensitive resin coated on the second planarizing layer 18 is patterned through exposure and development process to form the block stripe patterns 19. The block stripe patterns correspond to the photo diode regions 12.
On the entire surface including the block stripe patterns 19, a photosensitive film is coated and patterned through exposure and development process to form photosensitive film patterns 20 only on the block stripe patterns 19.
A reflow process is performed on the photosensitive film patterns 20 to form microlenses 20a. The microlenses 20a are formed on the block stripe patterns 19 as shown in FIG. 5b. Both the microlenses 20a and block stripe patterns 19 correspond to the photo diode regions.
The conventional CCD of FIG. 4 and the conventional method for forming the same as shown in FIGS. 5a and 5b, however, have the following problem which is illustrated in FIGS. 10a-1 and 10a-b.
As shown in FIG. 10a-1, the block stripe pattern 19 extends in one direction (along line E-E'). The microlens 20a covers the block strip pattern 19a so that the block strip pattern 19 extends through the middle of the microlens 20a. FIG. 10a-2 shows a portion of a cross-sectional view along line E-E' in FIG. 10a-1.
As shown in FIG. 10a-2, some of the light impinging on the microlens 20a is lost because the block strip pattern 19 is formed underneath in the middle of the microlens 20a. "P" represents the amount of light lost thereby. Therefore, although the radius of curvature difference between the major axis and minor axis may have been reduced, the light is lost and the performance of the conventional CCD is unsatisfactory.